How do you get polarized charged particles?

[BillKet]

Hello! how does one produced big ensembles of polarized charged particles (electrons, protons, muons etc.) for certain experiments? In the case of neutral particles (for example the nucleus in an atom) this could be done using a magnetic field, but I guess this won't work that straightforward with charged particles, as they would be affected by the Lorentz force. Thank you!

 

[Vanadium 50]

Depends on the particle. Electrons can be polarized by synchrotron radiation. Protons are normally polarized using atomic physics and the polarization preserved during acceleration. Antiprotons are produced via anti-Lambda decay.

 

[mfb]

Muons are produced polarized from pion decays (see e.g. the g-2 experiments).

 

[BillKet]

Depends on the particle. Electrons can be polarized by synchrotron radiation. Protons are normally polarized using atomic physics and the polarization preserved during acceleration. Antiprotons are produced via anti-Lambda decay.

Thanks! What exactly do you mean by "by synchrotron radiation"?

 

[Vanadium 50]

https://en.wikipedia.org/wiki/Synchrotron_radiation

 

[BillKet]

https://en.wikipedia.org/wiki/Synchrotron_radiation

I meant, how can you use synchrotron radiation to polarize electrons? Would any polarized light source work (e.g. a laser)? Or is there something special about the synchrotron radiation?

 

[Vanadium 50]

https://en.wikipedia.org/wiki/Sokolov–Ternov_effect

 

[bobob]

To get polarized pritions, you can do it the following way. First, the hydrogen hydrogen ground state has two levels that come from adding the proton and electron spins: The F=1 state which is 3-fold degenerate and the F=0 singlet state. So first, you start with H2 and dissociate it into atomic hydrogen and pass the atomic hydrogen through a nonuniform magnetic field. This spilts the F=0 and F=1 state into 4 components and since the electron magnetic moment is stronger, 2 of the 4 states are eliminated and the other passes through. However, what you have now is a beam with the electrons in the same spin direction, but the proton in different spin orientations, which is not what you want.

So, the next thing you do is pass the beam through a radio frequency "tansition unit" (which also has a magnetic field to keep the spins aligned) which flips the spins of the electron and proton so that now the protons are spin aligned and the electrons are not. The last thing you do is strip off the electrons and you are left with a polarized proton beam.

 

[Vanadium 50]

Except that you need to accelerate H- before stripping the electrons.

 

[bobob]

H- never enters the picture in this case. Up until the neutral hydrogen is ionized, the hydrogen is essentially moving at thermal velocities or more accurately, the velocity it attains by moving from a low pressure region (1 psi absolute into vacuum through a nozzle. There are other types of polarized sources, but this is the type of which I am most familiar.

 

[Vanadium 50]

In high energy accelerators, H- ions are accelerated in linear boosters that feed circular machines that carry protons. This is for the injection, where the injection trajectory is not the same as the orbit previously injected protons are on. Normally a thin foil does the stripping.

 

[bobob]

Ok, that is a method I am not familiar with. I am familiar with atomic ground state sources.